Overview
This lecture covers the origins, classification, textures, and compositions of igneous rocks, and how to interpret their geologic history and associated hazards.
Formation of Igneous Rocks
- Igneous rocks form from the cooling of magma (underground) or lava (at the surface).
- Classified as intrusive (inside the Earth, slow cooling) or extrusive (on Earth's surface, fast cooling).
Intrusive Igneous Rocks
- Form below Earth's surface, cooling slowly, allowing large crystals to form (coarse-grained texture).
- Examples: granite (light, silicic, continental crust), diorite.
- Crystal size is directly linked to cooling rate: slower cooling = larger crystals.
Extrusive Igneous Rocks
- Form on Earth's surface from lava cooling quickly, resulting in fine-grained texture.
- Textures: fine-grained (andesite), vesicular (skoria), glassy (obsidian).
- Very fast cooling traps gas bubbles or prevents crystal formation entirely.
Pyroclastic Materials
- Pyroclastic (tephra) materials are volcanic fragments ejected during eruptions.
- Includes volcanic ash (fine glassy fragments), lapilli ("little stones" or cinders), blocks (large solid fragments), and bombs (large, semi-molten, streamlined shapes).
- Ash can travel far; blocks and bombs often fall near the volcano.
Composition of Igneous Rocks
- Categorized as felsic, mafic, or ultramafic based on mineral content.
- Felsic: high in silica, low density, light color, explosive eruptions (e.g., granite).
- Mafic: rich in magnesium and iron, higher density, dark color, effusive eruptions (e.g., basalt).
- Ultramafic: very high magnesium and iron, low silica, very dense and dark, rare (e.g., peridotite).
Identifying Igneous Rocks (Examples)
- Granite: light-colored, coarse-grained, intrusive, felsic, forms continental crust, indicates slow cooling beneath the surface.
- Basalt: dark-colored, fine-grained, extrusive, mafic, forms oceanic crust, indicates rapid cooling at the surface.
Key Terms & Definitions
- Igneous Rock — Formed from cooled magma or lava.
- Intrusive (Plutonic) — Cooled slowly below Earth's surface, coarse-grained.
- Extrusive (Volcanic) — Cooled quickly on Earth's surface, fine-grained.
- Felsic — High in silica, light-colored, low density.
- Mafic — High in magnesium/iron, dark-colored, high density.
- Ultramafic — Extremely high in magnesium/iron, very dense, rare.
- Pyroclastic/Tephra — Fragmented volcanic materials ejected during eruptions.
- Vesicular Texture — Rock texture with gas bubbles.
- Glassy Texture — Rock with no visible crystals due to rapid cooling.
Action Items / Next Steps
- Review differences between intrusive and extrusive igneous rocks.
- Memorize key examples: granite, basalt, peridotite.
- Prepare for the next module on sedimentary rocks.